A continuously variable transmission for a vehicle is provided for example with a hydraulic transmission such as a torque-converter or a fluid coupling which transmits the engine rotation power, and a continuously variable transmission (referred to hereafter as CVT) which varies the rotation speed of the hydraulic transmission and transmits it to a vehicle drive shaft.
In Tokkai Sho 61-105353 published by the Japanese Patent Office in 1986, a CVT is disclosed wherein the speed change ratio is continuously varied by looping a belt around a drive pulley and driven pulley respectively comprising a V-shaped groove and varying the width of the pulley groove.
Oil pressure acts on each of these pulleys via a speed change control valve, and the width of the pulley is varied according to the oil pressure. In other words, the speed change ratio is varied by controlling the speed change control valve.
This oil pressure is supplied to the speed change control valve via a line pressure control valve from an oil pump driven by the engine. The speed change control valve comprises a spool valve connected to a step motor via a link. When the link displaces the spool according to an angular position of the step motor, the oil pressure acting on the pulley varies.
The speed change control valve for example simply transmits pressure from the oil pump to the driven pulley, only the oil pressure acting on the drive pulley being controlled according to a speed change ratio command signal. In this case, the higher the pressure acting on the drive pulley, the narrower is the groove width of the drive pulley, the larger is the contact radius between the drive pulley and the belt, and the smaller is the speed change ratio. When the pressure acting on the drive pulley is 0, the contact radius between the drive pulley and belt is a minimum and the speed change ratio is a maximum. When the vehicle is started, the oil pressure acting on the drive pulley is controlled to increase together with the rise of engine rotation speed, and the speed change ratio gradually decreases.
The step motor varies its angular position according to a speed change ratio command signal output from the control unit, and the spool of the speed change control valve is displaced via the link. The control unit calculates a target speed change ratio so that the engine rotation speed corresponds to a depression of an accelerator pedal, and outputs a corresponding speed change ratio command signal to the speed change control valve.
The displacement of this link is limited by a stopper at its maximum displacement position which corresponds to the valve position at which the valve releases the whole pressure acting on the drive pulley to a drain. The starting point for the angular position change of the step motor is set so that it overshoots an angular position at which the link comes in contact with the stopper, or an angular position at which the oil pressure acting on the drive pulley is completely opened to the drain, by a plurality of steps in the direction of increasing speed change ratio. When the vehicle is at rest, the step motor is held in this starting point position. The reason for performing this setting is that, if oil pressure were to act continuously on the drive pulley in the rest state, there would be an undesirable effect on the durability of the CVT.
Therefore, when the vehicle starts, an "initialization" must be performed wherein the step motor is rotated from this starting point to a true control starting position at which the speed change control valve actually begins speed change control. This task is performed when the control unit outputs a speed change ratio command signal for initialization.
However, due to scatter in the dimensions of mechanical parts which are interposed between output of the speed change ratio control signal and variation of groove width of the drive pulley, the step number of the step motor required for initialization is not necessarily the same for all transmission systems of the same specification. The case therefore arises where the angular position of the step motor may not coincide with the control start position even if initialization completes. When the angular position of the step motor does not reach the control start position, the response delay of the transmission increases during the accelerating process of the vehicle when it is started. This response delay adversely affects the acceleration performance of the vehicle. Conversely, when the angular position of the step motor has overshot the control start position, the vehicle starts with a speed change ratio which is smaller than the command value. Such a smaller speed change ratio also affects the acceleration performance of the vehicle.
The above initialization errors may also cause the real speed change ratio to overshoot the target speed change ratio during the acceleration after the vehicle is started.